PPH - Power Per Hour: This is the primary figure I base my army build queues around in the expectation that once all my Lost Arts and Building Upgrades are complete I should be able to maintain build queues around the clock.
PPU - Power Per Upkeep: Essentially how much power do I get for each unit of food consumed. The higher the value the more powerful army I can maintain. As I don't keep hold of an army for much beyond a week this figure has a lower priority. The less full your queues are the greater weight you should give to this value.
Pikemen - Somewhat surprisingly the first offensive unit you can build is the best. The only area it falls really short in is it's speed. At 1kph it is the slowest unit in your army but for BGs and Beacon attacks it matches or betters anything else in your army.
Paladins - When I first started playing I thought these were awesome, until I did the maths. Early game these can be better than your pikes but then they fall into a gap where pikes are better PPH and Seekers are better PPU.
Seekers - Fitting into the low consumption category are your seekers. If you are building an army for the long haul then you probably want to build these in your infantry slot. Note: They require Maranian Amulets which will probably prevent you from keeping these built constantly.
Pathfinders - A mixed unit falling somewhere between seekers and pikemen. If you are wanting to keep a constant queue of infantry these may be better than a mix of seekers and pikemen but only if you are struggling with food consumption, otherwise you'll find pikes better for their pph and ease of turnover.
Knight - Eventually superseded by Great Lords you will get a lot of use out of Knights. Their high pph and speed make them excellent for filling out your offence and giving you some fast strike options.
Great Lord - I'd hold off building lots of these until their Lost Art level eclipses your Knights, then switch over. Their better ppu makes it easier to hold a greater total power and their lower speed shouldn't be an issue by these point as you will likely have regular access to Wyverns or better to do your farming and rapid response.
Reaver - I'm not really sure why these exist. I don't have access to them yet so they may be incredibly cheap to build but other than that there are better units to be spending your resources and time on.
Priestess - These are your Maranian Amulet cavalry. Build these if you're not going to be attacking in the next week. Better than Seekers in ppu.
Warlock - The best of your occult offence. You get these before Necromancers, they are cheaper to upgrade than necros, they are a common reward from bgs and obviously have the best pph. Upgrade and enjoy.
Necromancer - Don't forget your necromancers' level entirely however. Even though I don't recommend building them you will still have a lot of them as they are the better choice of prize unit from tournaments. Level them up when you can.
Renegade - Maranian unit. These have a great ppu but will take you 10 hours of best rate Amulet production to have the rss to build just one.
Wyvern - While dragons are a better unit you will still have Wyverns coming into your army through various quest and bg rewards. Their speed of 4 makes them a nice rapid response force.
Dragon - Outside of Maranian units (and Eldritch units of course) these have the best ppu, and they also build at the maximum pph. These should be your go to bestiary unit assuming they are the same level as your Wyverns.
Chimera - Unless you have bought thieves these or Wyverns should be your farming army. I prefer these over Wyverns despite their higher upkeep per carry capacity because they are both faster and less likely to be accidentally included in an attacking force. It's easier to make sure an entire unit type is kept out of conflict than remember to subtract x amount each time you attack.